How are valuable metals recovered from the sludge left by electrolytic refining?

Valuable metals are recovered from the insoluble sludge—often called slimes—that accumulates at the bottom of electrolytic refining tanks through a multi-step process.12

First, the slimes are typically smelted in a furnace to oxidize and remove most base metals, leaving behind a metallic mixture known as doré. This doré, which contains silver, gold, and platinum-group metals, is then cast into anodes for further purification.13

The recovery of individual metals from this doré is achieved through two primary methods:

• Electrolytic Refining: The doré anodes are placed in an electrolytic cell containing a solution such as silver-copper nitrate. An electric current is applied, causing the silver to dissolve and deposit onto the cathodes in high-purity form.13
• Chemical Parting: Alternatively, the mixture can be treated chemically. For example, boiling the material in sulfuric acid or nitric acid can dissolve the silver, allowing the remaining gold to be filtered out as a residue.3

These processes ensure that precious metals, which do not dissolve during the initial copper refining, are efficiently captured as valuable by-products.14